The present invention relates to a method and apparatus for the control of an ignition system spark timing for a spark ignition internal combustion engine, and more particularly to a method and apparatus for the control of an ignition system spark timing within a warm-up period of the engine.
Description is now provided concerning the conventional art and the problems associated therewith. The spark timing control system for an internal combustion engine adjusts the timing of a spark to a certain degree before the top dead center (for example, 12.degree. BTDC/600 rpm) so as to cause the ignition to take place at the optimum instance to provide the maximum output because there is an ignition delay from the passage of a spark to an actual ignition of the fuel within the cylinder of the engine. The ignition delay in terms of time is substantially unchanged even if the engine revolution speed increases, but if it is considered in terms of crank angle, the ignition delay increases in accordance with an increase in the engine revolution speed because the piston speed increases in accordance with the increase in engine revolution speed. Thus, a centrifugal spark advance controller of the mechanical type, for example, is operatively connected to a distributor, thereby to advance the spark timing in accordance with an increase in the revolution speed because if the spark timing remains at the same level as that for the low revolution speed, an adequate output characteristic can not be provided when the speed increases.
FIG. 1 is a graph showing one example of a spark advance characteristic provided by the conventional centrifugal advance controller, wherein the axis of abscissa designates the engine revolution speed and the axis of ordinate indicates the spark advance value. The letter N.sub.1 designates a revolution speed beyond or above which the spark advance value increases, the letter N.sub.2 designates a revolution speed beyond or above which the spark advance value becomes maximum, the solid line designates the spark advance characteristic when the engine temperature is within its ordinary-in-use range, and the broken line designates the spark advance characteristic when the engine is within a low temperature range. This is often called a "two-point method" which is characterized by a selective use of two separate spark advance characteristics by switching in response to the low engine temperature range or the ordinary-in-use engine temperature range.
Induction vacuum as the engine is operating has a great influence on the ignition delay. Namely, since when the induction vacuum is great, i.e., when the throttle valve opening degree is small, the rate of residual gas within a cylinder is great and thus the density of fresh air fuel charge is low within the cylinder, thereby to cause a reduction in flame propagation, causing a drop in performance. As a countermeasure to this, it has been the conventional practice to provide a distributor with a vacuum spark advance controller.
FIG. 2 is a graph showing one example of a spark advance characteristic provided by the conventional vacuum advance controller, wherein the axis of abscissa designates the induction vacuum, the axis of ordinate designates the spark advance angle, the solid line designates the spark advance characteristic to be used when the engine temperature is within the ordinary-in-use temperature range, and the broken line designates the advance characteristic to be used when the engine temperature is especially low. The characteristic designated by the broken line shows that spark advance control due to induction vacuum is suspended during low engine temperature.
However, the conventional mechanical spark timing controls of the above mentioned types have the following problems. That is, in the case of the two point centrifugal spark advance controller which provides the two-stage spark advance characteristic as shown by the solid and broken lines in FIG. 1, it is necessary to effect switching in response to the temperature and it requires the addition of a point change unit, thus causing an increase in cost and a drop in reliability. Besides, since what is allowed by the characteristic is a parallel shift from one to the other only and thus a limited freedom, the problem of poor driveability cannot be completely solved with this limited freedom. At the same time, in the case wherein the spark advance control by the induction vacuum is suspended when the engine temperature is low, the spark advance value becomes very small when the engine temperature is low and there occurs a great difference between the spark advance when the vacuum control is effective and the spark advance when the vacuum control is suspended, thus resulting in poor driveability and fuel economy.
Recently, electronic spark timing controls have been developed which employ a digital processor, such as a microcomputer, to determine the optimum spark advance value in accordance with the engine revolution speed and load, and the electronic spark timing controls of the above type feature that the optimum spark timing suitable for every operating condition of the engine can be easily set, thus making a contrast to the conventional mechanical spark advance controller, such as centrifugal spark advance mechanism or vacuum spark advance mechanism.